This invention relates to fluid condition sensors or indicators intended for use in a heat exchanger system such as a radiator or air conditioning system.
A well known engine coolant for an automotive vehicle contains a solution of ethylene glycol and a small percentage of diethylene glycol diluted with water to make an approximately 50-50 mixture or lower depending on the desired freezing point for the solution. Generally, manufacturers or distributors of ethylene glycol add one or more corrosion inhibitors to the solution to protect the metal components of the engine cooling system, particularly the radiator. These inhibitors are usually a mixture of one or more inorganic salts such as phosphates, borates, nitrates, nitrites, silicates or arsenates and organic compound. The solution is usually buffered to a pH of 8 to 10 to reduce iron corrosion and to neutralize any glycolic acid.
It is very important that the coolant mixture in an engine cooling system contain 50 to 55% of properly inhibited ethylene glycol to prevent corrosion of conventional copper-brass radiators. Also, aluminum radiators have now come into use and such radiators are even more susceptible to corrosion. The problem of corrosion can be caused simply by the addition of ordinary water to the cooling system by a driver when he is low in coolant. For example, a reduction of the coolant mixture to 33% ethylene glycol and 67% water will increase metal corrosion significantly, especially in higher temperature coolant systems.
U.S. Pat. No. 4,338,959 issued Jul. 13, 1982 to Borg-Warner Corporation teaches a device for the automatic addition of corrosion inhibitor to a cooling system including an electronic control circuit having a probe or sensor which indicates corrosion rates in an engine cooling system and provides a signal to a solenoid activating a control valve which automatically adds a controlled amount of corrosion inhibitors solution to the cooling system. However, this known system has certain disadvantages, one of which is the amount of room required to install the system in a vehicle. For example, the system requires a container capable of holding an adequate reserve charge of corrosion inhibitor. Also, there is a significant expense involved in providing this system in a vehicle because of the cost of the corrosion inhibitor and other required components.
U.S. Pat. No. 4,306,127 issued Dec. 15, 1981 to Robertshaw Controls Company describes a corrosion sensor having a housing containing an electric switch mechanism. A switch actuator is held in one operating position by a corrosion sensing member formed from a disk of aluminum foil that spans one end of the housing. The actuator moves to another switch operating position when the aluminum foil ruptures through the corrosion thereof caused by being exposed to a corrosive material. A flexible diaphragm is carried by the housing in stacked engaging relation with the aluminum disk and acts to prevent corrosive material from entering the housing when the aluminum disk ruptures.
U.S. Pat. No. 4,736,628 issued Apr. 12, 1988 to V. S. Lynn describes a testing device for a car battery or radiator, which device includes a transparent box-like housing forming a partitioned chamber. There are a plurality of channels in which a plurality of balls having various densities are contained. The fluid to be tested enters the housing through a tubular element at the top. The number of floating balls in the housing indicates the specific gravity of the fluid and the freezing and boiling point thereof.
Test results reported in a paper co-authorized by one of the present inventors (Brian Cheadle) confirm that relatively high degrees of coolant depletion are required to initiate corrosion damage in aluminum cylinder head material compared to corrosion of a corrodible diaphragm. This paper entitled "Controlled Release of Inhibitors for Extended Protection of Aluminum Engine Cooling Systems" was published by the Society of Automotive Engineers as Paper 820287 in 1983.